Certain variations can only appear at corresponding ages;
for instance, peculiarities in the caterpillar, cocoon, or imago states of
the silk-moth; or, again, in the full-grown horns of cattle. But
variations which, for all that we can see might have appeared either
earlier or later in life, likewise tend to reappear at a corresponding age
in the offspring and parent. I am far from meaning that this is invariably
the case, and I could give several exceptional cases of variations (taking
the word in the largest sense) which have supervened at an earlier age in
the child than in the parent.
These two principles, namely, that slight variations generally appear at a
not very early period of life, and are inherited at a corresponding not
early period, explain, as I believe, all the above specified leading facts
in embryology. But first let us look to a few analogous cases in our
domestic varieties. Some authors who have written on Dogs maintain that
the greyhound and bull-dog, though so different, are really closely allied
varieties, descended from the same wild stock, hence I was curious to see
how far their puppies differed from each other. I was told by breeders
that they differed just as much as their parents, and this, judging by the
eye, seemed almost to be the case; but on actually measuring the old dogs
and their six-days-old puppies, I found that the puppies had not acquired
nearly their full amount of proportional difference.
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